Heretofore, as high polymer materials having rubber-like elasticity, those obtained by formulating a crosslinking agent, a reinforcing agent, and the like in various rubbers, such as natural and synthetic rubbers, and crosslinking the resulting compositions at high temperature and high pressure have been used broadly. However, such rubbers require prolonged crosslinking and molding under high-temperature, high-pressure conditions, thus being poor in processability. Moreover, the crosslinked rubbers do not exhibit thermoplasticity so that, unlike thermoplastic resins, recycle molding is generally infeasible. For this reason, recent years have seen the development of several recyclable thermoplastic elastomers which may be easily reprocessed into shaped articles by utilizing the universal melt-molding techniques such as hot-press molding, injection molding and extrusion molding as it is the case with ordinary thermoplastic resins. As such thermoplastic elastomers, several polymers in the olefin, urethane, ester, styrenic, and vinyl chloride series have been developed and are on the market today.
Among these polymers, styrenic thermoplastic elastomers are highly flexible and exhibit satisfactory rubber-like elasticity at atmospheric temperature. As such styrenic thermoplastic elastomers, styrene-butadiene-styrene block copolymer (SBS), styrene-isoprene-styrene block copolymer (SIS), and the corresponding hydrogenated elastomers such as styrene-ethylenebutylene-styrene block copolymer (SEBS) and styrene-ethylenepropylene-styrene block copolymer (SEPS) are known. However, these block copolymers are inadequate in permanent compression set.
Meanwhile, as a thermoplastic elastomer having good flexibility, exhibiting good rubber-like elasticity at atmospheric temperature, and having satisfactory vibration damping properties, an isobutylene block copolymer comprising a polymer block composed predominantly of isobutylene and a polymer block composed predominantly of an aromatic vinyl compound is known (U.S. Pat. No. 4,276,394). However, this isobutylene block copolymer has also proved unsatisfactory in the degree of compressive deformation on heating (permanent compression set) and in rubber-like elasticity at high temperature.
Also known is a thermoplastic polymer composition comprising a crosslinking product of an isobutylene block copolymer component containing a polymer block composed predominantly of isobutylene and a rubber component (WO98/14518). This composition is outstanding in gas barrier and sealing properties but has been found to have the drawback that it is still unsatisfactory in permanent compression set, giving a value of 35 to 65 under the condition of 70° C.×22 hours.
Thus, there has not been known a thermoplastic elastomer satisfactory in all of moldability, permanent compression set, and vibration damping properties.